1. Field of the Invention
The present invention generally relates to the measurement of the level of fluids such as oil, in certain types of tanks in refineries and oil terminals. The present invention is more particularly related to the use of radar for the measurement of the level of the fluid in tanks which until now have been impossible to measure directly with radar.
The tanks under consideration are fitted with floating roofs, but in contrast to many other such tanks, constructed without any fluid or oil surface accessible for radar level gauging. The reason why no fluid or oil surface is available for radar level gauging may be a result of government environmental requirements (e.g., to avoid the leakage of hydrocarbon vapor). This is particularly true over the past decade where environmental requirements have become more important and more regulatory demanding. Alternatively, the floating roof tank may not be fitted with a still pipe which is generally used for level gauging.
2. Background of the Related Art
Level gauging devices have become more and more important particularly for petroleum products such as crude oil and products manufactured therefrom which are stored in containers or tanks. By containers it is meant in this context large containers constituting parts of the total loading volume of a tanker, or even larger usually circular-cylindrical land-based tanks with volumes of tens of thousands of cubic meters.
A typical tank is 20 m high and 40 m in diameter making a volume of 25000 cubic meters. Floating roof tanks are generally bigger than fixed roof tanks. With 100 tanks at one location, the demand for automatically reading the level of fluid in the tank is particularly important considering that the 100 tanks must be dispersed over an area of one square mile. Accuracy of measurement of the fluid level is very important since a few milimeters in the fluid level in the tank is generally worth more than the level gauging equipment itself. As a result, high demands are placed on the gauging equipment which are used to determine the fluid level in the containers.
Radar has been used for level gauging during the last 20 years (see U.S. Pat. Nos. 4,044,355 and 4,665,403 and many others) and has been generally used on refinery tanks or containers for noncontact measurements of the fluid level contained therein with high accuracy. The classical application is in a big tank with fixed roof where the radar beam passes through the entire empty space of the container which is unoccupied by the oil down to the oil surface or some other liquid surface.
Since the velocity of radar waves transmitted through air or other gases is generally stable, accurate measurements of the fluid level may generally be obtained. In addition, level measurements can be employed under adverse conditions resulting from extreme temperatures, chemical corrosion and mechanical stress because the radar antenna can be made of very durable material. Further, since the radar antenna can be mounted in a hole in the top of the container or tank, its installation becomes simple. Maintenance and eventual replacement of the radar antenna is also relatively simple due to its accessible location on the top of the tank.
Another common application of radar level gauging applies to an oil tank with a floating roof where no substantial empty space occurs for the radar beam to measure the oil beneath the floating roof. The roof floats on the oil like a raft with a sliding seal against the tank walls, preventing the oil and associated gases from escaping. Due to wind pressure and friction, the position of the roof itself is a poor measure of the position of the oil surface if an accurate measurement of 1-2 mm is required. Thus, level gaging utilizing the position of the roof has not been traditionally used for accurate level gauging.
Most tanks with floating roofs are therefore fitted with a standard still pipe which is slotted passing through a hole in the roof to get a place to measure the level. The still pipe requires a large diameter of 6"-20" and has holes or slots therein to guarantee the same oil level inside the pipe as outside. Level gauging can generally be performed using the still pipe which may also be used for taking samples for laboratory tests. One of the classical uses for this pipe is to protect a mechanical float gauge.
One method of level measuring which has been employed is to guide the radar waves in a wave guide that extends downward through the tank. Generally, a narrow diameter of the wave guide is required to properly transmit the radar signal from the top of the tank through the entire length of the wave guide. Level measuring according to this method has heretofore been tried, but serious practical limitations relating to the required small diameter of the wave guide to be suitable for the radar frequency range have been experienced. The wave guides that are referred to have comprised rectangular or circular cylindrical pipes of metal with dimensions that allow one-mode propagation. See, for example, U.S. Pat. No. 4,359,902, incorporated herein by reference.
The narrow diameter wave guide generally presents the following problems: First, if the crude oil is rich in wax, the pipe becomes easily clogged. Second, the propagation of the radar waves is unacceptably influenced by the hole in the tubular wave guide that is needed for assuring free flow of liquid between the fluid contained in the tank and the fluid contained inside of the wave guide. Third, corrosion in the pipe causes unacceptable damping in transmission from the top of the tank to the bottom for standard tank heights. It therefore becomes necessary to make the pipe of expensive material or to coat its inside with noble metal to prevent such corrosion.
U.S. Pat. No. 4,641,139 (hereinafter '139 patent), incorporated herein by reference, attempts to overcome the requirement of a narrow wave guide by employing a powerfully over-dimensioned wave guide to which radar radiation is so conducted that all undesired wave guide modes are suppressed. The wave guide in the '139 patent, in the majority of practical cases, preferably consists of an existing pipe in the cistern or tank, requiring the radar level gauge to tolerate substantial variations of the dimensions of the wave guide pipe, depending on the size of the existing pipe in the tank.
However, in the last few years, the use of these slotted still pipes has been reconsidered or reevaluated in some areas (e.g., the states of Texas and California in the United States) because the still pipes are considered to leak too much hydrocarbon vapor. The still pipes may, for that reason, be sealed in accordance with governmental regulations, and thus, are impossible to be used in level gauging. Further, it has been discovered that since these still pipes are required to extend the entire height of the tank, the still pipes are susceptible to adverse conditions in the tank and may not always provide the best medium for transmitting the radar waves from the top of the tank to the floating roof.
Accordingly, it is desirable to measure the level of a fluid in a tank without the general problems experienced by previous systems and methods. It is also desirable to measure the level of a fluid in a tank without the constraints of a still pipe which may leak hydrocarbon vapor. It is further desirable to measure the level of a fluid in a tank without the problems associated with a still pipe which extends along the height of the tank.